Energy selective ion beam intensity measuring apparatus and method utilizing a scintillator to detect electrons generated by the beam

ABSTRACT

The apparatus is primarily intended for use with ion beams from mass spectrometers, and has one application in determining metastable ion spectra. It comprises a retarding electrode carrying a scintillator detector and preceded by an apertured electrode. When the retarding electrode voltage equals or exceeds the source accelerating voltage, all ions are repelled to the apertured electrode where secondary electrons are produced which are accelerated to the scintillator and an output is obtained from a photomultiplier located beyond the retarding electrode. When the retarding electrode voltage is less than the source accelerating voltage only ions resulting from metastable fragmentations (which have less energy) are so repelled. The parent ions strike the scintillator but produce no output because the latter is coated with an ion-opaque layer. A second, apertured, retarding electrode can precede the above-described arrangement, to which a lower voltage is applied, thus setting a lower as well as an upper limit to the ion energy detected and making the apparatus of more general application as an energy range selector.

United States Patent [72] inventors Norman Richard Daly Reading; I RoyalEdward Powell, Newbury, England {2]} AppLNo. 769,644

(22] Filed Oct. 22,1968

[45] Patented May 18, 1971 [73] Assignee United Kingdom Atomic EnergyAuthority London, England [32] Priority Oct. 31, I967 [33] Great Britain[54] ENERGY SELECTIVE ION BEAM INTENSITY MEASURING APPARATUS AND METHODUTILIZING A SCINTILLATOR TO DETECT ELECTRONS GENERATED BY THE BEAMAssistant Examiner-C. E. Church AttorneyLarson, Taylor and HindsABSTRACT: The apparatus is primarily intended for use with ion beamsfrom mass spectrometers, and has one application in determiningmetastable ion spectra. it comprises a retarding electrode carrying ascintillator detector and preceded by an apertured electrode. When theretarding electrode voltage equals or exceeds the source acceleratingvoltage, all ions are repelled to the apertured electrode wheresecondary electrons are produced which are accelerated to thescintillator and an output is obtained from a photomultiplier locatedbeyond the retarding electrode. When the retarding electrode voltage isless than the source accelerating voltage only ions resulting frommetastable fragmentations (which have less energy) are so repelled. Theparent ions strike the scintillator but produce no output because thelatter is coated with an ion-opaque layer. A second, apertured,retarding electrode can precede the above-described arrangement, towhich a lower voltage is applied, thus setting a lower as well as anupper limit to the ion energy detected and making the apparatus of moregeneral application as an energy range selector.

Patented May- 18, 1971 I 3,579,270

6 Sheets-Sheet 1 Patented May 18, 1971 I 3,579,270

a Sheets-sheaf 2 Patented May'18,- 1971 3,519,270

6 Sheets-Sheet 4 fm warm 7 v 2'0 4 0. a? 12 m? z) w 45% 7 Patented May18, 1971 3,519,270

6 Sheets-Sheet 6 ENERGY SELECTIVE ION BEAM INTENSITY MEASURING APPARATUSAND METHOD UTILIZING A SCINTILLATOR TO DETECT ELECTRONS GENERATED BY THEBEAM BACKGROUND OF THE INVENTION This invention relates to ion beamintensity measuring apparatus and methods suitable for massspectrometers and has one application in measuring the metastablefragment ion content of such a beam. More generally, the presentinvention enables ions below a predetermined energy to be selected fordetection. and in one form enables ions within a predetermined energyrange be selected for detection.

In the mass spectrometry of organic molecules, the ions formed in theion source consist of ions of the parent molecule, together withfragment ions formed from parent molecules which are fragmented in theionizing process; the fragment ions fonned in ionizing a given organicmolecule can have numerous different chemical compositions.

Some of the parent and fragment ions formed at the source aremetastable, and fragment spontaneously during the flight of the ionsbetween the ion source and the ion beam intensity measuring system. Theionized products of such spontaneous fragmentations are herein termedmetastable fragment ions.

Considerable information about the structure of the parent molecule canbe obtained by examining the metastable fragment ion spectrum, buthitherto it has been difficult to separate out the metastable fragmention content from the other ions in the spectrum.

It is one object of the present invention to provide apparatus whichenables the metastable fragment ion content to be determined morereadily than heretofore.

SUMMARY OF THE INVENTION According to the present invention ion beamintensity measuring apparatus suitable for use with a mass spectrometercomprises a first apertured electrode for admitting the ion beam, afirst retarding electrode located beyond said first apertured electrodeto apply a retarding electric field to ions passing through theaperture, a detector for detecting secondary electrons emitted from saidfirst apertured electrode by ions repelled to said first aperturedelectrode by said field, and a connection for applying a variablepotential to said first retarding electrode.

Said secondary electron detector may be mounted on said first retardingelectrode facing said first apertured electrode. The surface of saidsecond electron detector facing said first apertured electrode mayinclude a coating, which may be metallic, which is substantially opaqueto said ions but substantially transparent to said secondary electrons.Said detector may be a translucent scintillator and form the centralportion of said first retarding electrode. The photocathode of aphotomultiplier tube may be located beyond said first retardingelectrode opposite that surface of said scintillator which faces awayfrom said first apertured electrode.

Said first apertured electrode may be preceded by a second retardingelectrode having an aperture and having a connection for applyingthereto a variable potential less than the potential applied to saidfirst retarding electrode. Said second retarding electrode may include agrid.

Said first apertured electrode may be preceded by a second aperturedelectrode arranged to be held at the same potential as said firstapertured electrode, an apertured electron-suppression electrode beinglocated between said first and second apertured electrodes.

Said first apertured electrode may be preceded by a second aperturedelectrode arranged to be held at the same potential as said firstapertured electrode, an apertured electron-suppression electrode beinglocated between said first and second apertured electrodes, said secondretarding electrode preceding said second apertured electrode, and athird apertured electrode preceding said second retarding electrode andarranged to be held at the same potential as said first and secondapertured electrodes.

The first retarding electrode and the first apertured electrode aspreferably substantially parallel to one another.

Also according to the present invention a method of measuring ametastable fragment ion spectrum with a mass spectrometer comprisesusing an apertured electrode to admit the ion beam, a retardingelectrode located beyond the apertured electrode to apply a retardingelectric field to ions passing through the aperture, a detector mountedon said retarding electrode to detect secondary electrons emitted fromsaid apertured electrode by ions repelled to said apertured electrode bysaid field, and varying the potential applied to said retardingelectrode between a potential at which substantially all ions in thebeam are repelled to said apertured electrode and a potential at whichsubstantially only metastable fragment ions in the beam are so repelled.

Further according to the present invention a method of measuring theintensity of ions in a beam within a desired ion energy range comprisesusing an apertured electrode to admit the ion beam, a first retardingelectrode located beyond the apertured electrode to apply a retardingelectric field to ions passing through the aperture, a detector mountedon said first retarding electrode to detect secondary electrons emittedfrom said apertured electrode by ions repelled to said aperturedelectrode by said field, a second retarding electrode having an apertureand preceding said apertured electrode to apply a retarding field toions approaching said second electrode, the potential applied to saidsecond retarding electrode being smaller than the potential applied tosaid first retarding electrode and said potentials being selected todefine between them the desired ion energy range.

DESCRIPTION OF THE DRAWINGS to the accompanying drawings wherein,

FIG. 1 is a simplified diagram of a mass spectrometer embodying one formof the invention.

FIG. 2 is diagram similar to FIG. 1 illustrating another form of theinvention.

FIG. 3 is a sectional elevation of an ion beam intensity measuringapparatus of the kind illustrated in FIG. 2.

FIG. 4 shows graphs of log (ion current) against first retardingelectrode voltage for various types of ions, with first retardingelectrode voltage (V accelerating voltage (V -AV.

FIG. 5 shows a normal" spectrum of trans-Z-butene with V,=V,, +AV.

FIG. 6 shows the spectrum of metastable fragmentations of trans-Z-butenewith V,=V,,AV, where AV==V /9O.

FIG. 7 is a graph of ion current versus second retarding electrodevoltage for (m/e)55 in trans-Z-butene.

Referring to FIG. 1, a mass spectrometer comprises a conventional ionsource 1 which is maintained at a high positive potential V in thepresent example +6 kv. An organic compound e.g. a complex hydrocarbon,introduced into the source in gaseous form is ionized by an electronbeam 2 to produce positive ions which are extracted from the source andaccelerated towards an electrode 3 having a slit and maintained at earthpotential. The ion beam 4 emerging from the slit is deflected by anelectromagnet 5 whose variable field is normal to the plane of thedrawing, in a manner familiar to those skilled in the mass spectrometerart.

The beam emerging from the magnet 5 first passes through a slit in anearthed electrode 6 (constituting the aforementioned second aperturedelectrode), followed by a wide slit in an electrode 7 which ismaintained at a potential of approximately l00 v. to suppress secondaryelectrons formed by any ions which strike electrode 6. The electron-freebeam next passes through an earthed electrode 8 (constituting theaforementioned first apertured electrode) having a slit wider than thatin electrode 6 to prevent the emission of secondary electrons bycollision of the beam with its edges, beyond which is an annular firstretarding electrode 9 whose center is occupied by a glasslikescintillator 10. eg a CaF (Eu-loaded) crystal. Beyond electrode 9 is avacuum-tight glass window 11, against the other side of which (i.e.outside the vacuum chamber of the mass spectrometer) is mounted thephotocathode 12 of a photomultiplier tube. Electrode 6 forms theresolving slit of the spectrometer in this embodiment.

As mentioned in the introduction, the ion beam 4 approaching electrode 6consists of parent ions, fragment ions and metastable fragment ions,these last having been largely formed during the passage of the beambetween electrodes 3 and 6. The electrode 9 is maintained at a potentialwhich is variable between a potential V,r slightly positive to theaccelerating potential V, of source 1, say +6.1 kv., and a potentialslightly negative to the potential of source 1, say +5.9 kv.

When electrode 9 is at +6.1 kv. all the ions traversing the slit inelectrode 8 are turned back towards earthed electrode 8, and strike itssurface to produce secondary electrons which are accelerated into theglass scintillator 10. The resulting light is measured by thephotomultiplier and is a measure of the parent, fragment and metastablefragment ion content of the beam.

When electrode 9 is at 5.9 kv., the parent and fragment ions approachscintillator l and strike it with sufficient energy .to produce a signalin the photomultiplier. However the metastable fragment ions, whoseenergy is lower than that of those parent and fragment ions which didnot fragment between electrodes 3 and 6, are repelled to electrode 8,where they produce secondary electrons which are accelerated toscintillator 10 as before. Hence the photomultiplier output in thisinstance is a measure of the metastable fragment ion intensity only.

To assist in discriminating between low-energy ions and secondaryelectrons, the surface of scintillator 10 which faces electrode 8 iscoated with a thin layer of aluminum (not shown), of thickness a fewpg/cmF, which is substantially opaque to the ions but transparent to thesecondary electrons. It is desirable that electrodes 8 and 9 should besubstantially parallel to one another as shown, in order to maintain theuniformity of the field between them.

It will be understood that the :100 v. variation, AV, of electrode 9relative to the source voltage, as described above, is not necessarilythe most suitable variation for all measurements, this being a matterfor experimental adjustment.

Various modifications of the described apparatus are possible. Forexample the electrode 9 and the scintillator 10 may be placed in contactwith the inner surface of window 11.

The variable potential for electrode 9 is conveniently obtained from apotentiometer across a voltage source which also supplies the potentialfor source 1.

If the fragmentation process occurs between electrode 3 and magnet 5(flight portion 13), it can be shown that the metastable fragment ion M;formed in the spontaneous fragmentation process M, M another fragment,usually neutral, which can be ignored for present purposes),

an apparent or pseudo mass M* where The fragmentation patterns that canbe seen in the usual spectrum consist of various M values, which can benonintegral and usually form broad peaks. Usually a large metastablefragment ion M* is about 1 percent of the intensity of the most intensepeak; moreover it can lie under a large peak (due to an ion formed inthe initial ionization process) and be dif-,

through the magnet, but these are scattered over a wide mass range ofthe spectrum and cannot be analyzed. Spontaneous fragmentation alsooccurs between magnet 5 and the detector, but hitherto this has not beendetectable because the fraction of metastable fragment ions M2+ soformed in the process Mr My (+another fragment, as above) is swamped bythe much larger signal from M The present invention enables the M ionsto be rejected and hence the M ions to be detected.

Thus the present invention enables spontaneous fragmentation occurringover two portions of the flight to be examined, viz. between electrode 3and magnet 5 (flight portion 13), and also between magnet 5 andelectrode 6 (flight portion 14). Moreover, the measurements relating toportion 14 will indicate immediately whether M, is metastable, whereaswhen interpreting those relating to portion 13, it is necessary tomanipulate numbers with a knowledge of organic chemistry to decide thatM* really derives from the fragmentation of M,* and not from some otherion Mf.

DESCRIPTION OF PREFERRED EMBODIMENT FIG. 2 shows a form of the inventionwhich can be used not only for the measurement of metastable spectra,but more generally to detect ions within a predetermined energy range.In FIG. 2, in which numerals ll2 indicate the same features as in FIG.1, there is provided a further apertured electrode 15 (constituting theaforementioned second retarding electrode), in the form of a grid;alternatively a slit may be used. Electrode 15 precedes aperturedearthed electrode 6 and is preceded in turn by an apertured earthedelectrode 16 (constituting the aforementioned third aperturedelectrode). Electrode 16 forms the resolving slit of the spectrometer inthis embodiment.

In the arrangement described with reference to FIG. 1, the potential V,-applied to electrode 9 was varied between values slightly above andslightly below the source potential V,,, depending on whether all ions,or only metastable fragment ions, were to be detected, respectively. Inthe arrangement of FIG. 2, the potential of electrode 9 is again shownas +V and the source accelerating potential V is again +6 kv. in thisexample. The electrode 15 has applied to it a potential +V,, which isless than +V and depends on the range of ions it is desired to select.

In operation, ions leaving the magnet 5 with energy less than eV do notpenetrate beyond electrode 15 because of the retarding field appliedbetween electrodes 15 and 16. Those ions which do penetrate are restoredto their initial energy by the accelerating field existing betweenelectrode 15 and electrode 6. Of these ions, those with energies lessthan eV are repelled to electrode 8 where they produce secondaryelectrons which are detected by scintillator 10 in the manner describedwith reference to FIG. 1. Ions with energy greater than eV strikescintillator 10, but with insufficient energy to penetrate the thinlayer of aluminum (not shown) which covers the scintillator. This layeris substantially opaque to ions up to quite high energies (severalkev.), but transparent to the secondary electrons, as already described.Thus variation of +V and +V,, enables one to measure the abundance ofions in an energy band of width eV eV,, over a range of energies up tothe maximum determined by the source potential.

The arrangement of FIG. 2 can be applied to spark-source massspectrometers or sputtering-ion source mass spectrometers, to replacethe electrostatic analyzers normally used with these instruments. It canalso be used in other than mass spectrometer applications, e.g. toexamine the energies of ions from a laser-produced plasma.

In FIG. 3 a stainless-steel body 17 is closed at one end by a flange 18.Between body and flange is located a glass plate 111, corresponding towindow 11 in FIG. 2, an O-ring vacuum seal being provided between plate111 and body 17 as shown. The photocathode 112 of a photomultiplier tubeis mounted against the outer surface of plate 111. At the other end ofthe body is a circular aperture 19 for admitting the ion beam from theanalyzing magnet of the mass spectrometer. Face 20 of the body isadapted to be mounted in a vacuum-tight manner against the vacuumchamber of the spectrometer, using screwed holes 21.

A locating block 28 is bolted to the inside of body 17 and has acircular aperture 22 in register with aperture 19. Secured to therecessed upper surface of block 28 is a circular plate 116 having acentral knife-edge slit 116' which is threeeighths inch long and thouwide (9.S 0.25 mm.). Plate 116 10 corresponds to earthed electrode 16inFIG. 2.

Mounted on three steel pillars 23 is a rectangular plate 106 having aknife-edge slit 106' of the same dimensions as slit 116'. Plate 106corresponds to electrode 6 in FIG. 2. Secured to a ring-shapedprojection on plate 106 is a circular plate 108 having a knife-edge slit108 which is one-half inch long and 30 thou wide (12.7X0.75 mm.). Plate108 corresponds to electrode 8 in FIG. 2.

Between plates 106 and 108, and mounted on plate 106 by means of twoinsulating bushes, is a circular plate 107 having a slit 107' which isfive-eighths inch long and 0.1 inch wide (I5.9 2.54 mm.). Plate 107corresponds to electrode 7 in FIG.2.

Mounted on three insulating glass pillars 24 (of which only the rearmostis seen in this section) are two steel blocks 25 (of 25 which only therearmost is seen) carrying a rectangular plate 109. Plate 109 has acentral aperture of three-eighths inch diameter (9.5 mm.) in which ismounted a CaF (Eu) scintillator 110, these items corresponding toelectrode 9 inch scintillator 11 respectively in FIG. 2.

Extending downwards in the drawing from blocks 25 are three insulatingglass pillars 26 (of which only the two rearmost are seen) supporting arectangular plate 115 having a hole 115' which is five-eighths inch indiameter (15.9 mm.) and within which two parallel fine wire mesh grids(not shown) are held by rings 27. The grids have 375 squares per inch(14.75 per mm.). Plate 115 corresponds to electrode 15 in FIG. 2.

The spacing between the electrodes can be seen from the drawing, whichis approximately to scale, the internal diameter of body 17 being 4%inches (1 1.4 cm.). All the plates are of stainless steel.

Electrical connections to plates 107, I09 and 115 are made by wires (notshown) taken to lead-through connections (not shown) sealed into thewalls of the body 17. Plates 116, 106 and 108 are earthed to body 17 bytheir respective modes of mounting.

In FIG. 3 slit 116 forms the resolving slit of the mass spectrometer andthe ion beam is arranged to focus thereat. If

desired, an alternative resolving slit, e.g. of adjustable width, canprecede plate 116. Such an alternative slit (not shown) can be mountedin the recess in locating block 28, and the ion beam focus alteredaccordingly.

FIGS. 47 show results obtained with the form of apparatus shown in FIGS.2 and 3. The behavior of three types of positive ion in such apparatusmay first be considered with reference to FIG. 2.

Type I-Stable ions: These pass through electrode 8, with kinetic energyeV,, volts where V is the potential drop between the ion source 1 andelectrode 3. At some point between electrodes 8 and 9 where thepotential is V the kinetic energy of these ions is e(V,,V The ions willbe turned back to electrode 8 if the kinetic energy is zero, i.e., at

a point between electrodes 8 and 9 of potential V where V,

=V In other words to record a normal mass spectrum the potential V ofthe scintillator must be at least equal to V Type II-Metastable ionsfragmenting before electrode 8. If the decomposition If V,- is less thanV but greater than (M-JM -V then normal ions are rejected and onlyfragment ions from metastable fragmentations with energies less thanMg/M] -b-eV are recorded as described above.

Type III-Metastable ions fragmenting between electrodes 8 and 9. Assumethat the parent ion MR passes through electrode 8 with kinetic energy eVIts kinetic energy at a point of potential V nearer electrode 9 is thuse(V V,). Now suppose M fragments to M-j' plus a neutral fragment. At theinstant of its formation the kinetic energy of My is Mz/M be(V, -B1 At apoint V of higher potential, the kinetic energy will be Mg/M -be(V,iV)e(V V and the ion will be turned back to electrode 8 if this is equalto zero, i.e. if

M2 M2 M.) M?- In other words these fragment ions are not turned back andthus recorded at a discrete value of potential, but may have energiesranging from M /M1 'b-eV up to eV depending on the potential V, at thepoint of their formation.

The results shown in FIGS. 47 were obtained with a single-stage 12-inchradius 90 sector mass spectrometer. This instrument had a conventionalNier-type electron-impact ionsource.

FIG. 4 illustrates the effect of varying V, on the intensities of thethree types of ion beams described above. Curve C is of mass 40 in thespectrum of argon (i.e. a stable ion of type I as described above). WithV,=V (in this case 4.5 kv.) all ions entering the measuring apparatusare turned back to electrode 8 and thus recorded. As V is reduced, theions begin to strike the aluminum foil and thus the recorded ion currentdrops. At V,-=(V 30) V, the ion current is reduced by a factor of 6000.This represents the maximum suppression of normal ions. As V is furtherreduced a continuous rise in the measured ion current is observed. Thisis probably due to optical effects resulting from pinholes in thealuminum coating in the scintillator 10 as ions of increasing energyimpinge on the thin metal coating. Curve D was obtained by adjusting themagnet 5 to focus (m/e) 54 of trans-2-butene at the resolving slit. Thismass-to-charge ratio comprises in the main two parts. Firstly there arestable (type I) ions of mass 54 formed in the source ionization chamberand thus entering the scintillator region between electrodes 8 and 9with source energy. A sharp cutoff in the detection of these ions isagain recorded when V,- (V 20) V. A second part of (m/e) 54 comprisesions of mass 55 formed by metastable fragmentation of mass 56 in flightportion 13 of FIG. 1. These ions have energy 55/56'eV and appear in thespectrum at (m/e) 54.01. They pass through electrode 8 with energy55/56-eV, and are thus detected so long as V is greater than 55/56-VWhen V is reduced below this value a sharp cutoff again occurs since theions all have the same energy when they enter the scintillator region.The parent ion, (m/e) 56, of trans-2-butene produces all three types atthe measuring apparatus as shown by curve A of FIG. 4. Ions notdecomposing before or in the apparatus and having energy eV,, are againsharply cutoff when V,- V Fragment ions M formed between electrodes 8and 9 (type III as described above) have energies ranging between MJ56-Vand eV and show a gradual cutoff as V is decreased from V A to (V,,Mj56'V The sharp cutoff at (V V is due to abrupt nonrecording of ions ofmass 55 formed by the transition 56 55 *in flight portion 14 of FIG. 1.These enter the scintillator region between electrodes 8 and 9 with adiscrete energy 55/56'eV (equal in this particular case to (V -8O) eV.Similar arguments apply to curve B which is of (m/e) 55 intrans-Z-butene.

FIG. 5 is a reproduction of the mass spectrum of trans-Z-butene scannedfrom (m/e) 20to (m/e') 58 by variation of the field of magnet 5. V was 8kv. and V was 20 volts higher, i.e. sufficient to turn back all ions inthe spectrum to electrode 8. The spectrum ,was recorded at a singlegain-setting on a pen recorder. Under such conditions the intensity ofpseudo-mass peaks was too low for detection. Now the spectrum wasrescanned at 100 times greater sensitivity with V,=(V 90)V and theresults of FIG. 6 were obtained. Ions of energy eV are now buried in thealuminum coating and are not recorded. All fragment ions formed inmetastable fragmentations occurring between electrodes 3 and 16 aredetected. Those formed in flight portion 13 produce the well-known broadpeaks which are here referred to as pseudo-mass peaks. The absence ofthe normal ion beams allowed a much greater measurement sensitivity tobe used, and in the mass range scanned for FIG. 6 many more pseudo-masspeaks were observed than had previously been reported.

In addition to pseudomass peaks, FIG. 6 shows sharp peaks occurring atintegral (m/e) values. These are due to the products of metastablefragmentation occurring after magnetic mass dispersion of the metastableion (i.e. occurring in flight portion 14 of FIG. 1) and are measuredsimultaneously at the mass-to-charge ratio of this ion. Such fragmentsare not amenable to detection by conventional methods owing to thepreponderance of normal ions at the same (m/e) value. By scanning theretarding voltage V,, applied to electrode 15, the masses of thesemetastable fragment ions can be determined. FIG. 7 shows the resultsobtained for these metastable fragment ions occurring at mass 55 in thetrans-2-butene spectrum. V was set so that only ions resulting fromfragmentation were detected. V was increased from earth up to V Steps onthe V versus ion current curve correspond to rejection of ions of energyless than eV If the mass focused at the resolving slit 16 is M then themass of the rejected fragment M is given by The three steps shown inFIG. 7 correspond to the fragmentations C,,H,-C H "H and the finalcutoff at V =V corresponds to rejection of the residual normal" ions ofmass 55. Pseudo-mass peaks at (m/e) =l5.3, 27.7, and 51.07 confirm thethree transitions described. The large increase in ion current justbefore the final cutoff is probably due to defocusing of the ion beam onto the edges of the slit in electrode 8, with consequence release ofsecondary electrons. In other words a very large signal due to thenormal ion beam as well as that of the less energetic fragments is beingrecorded in spite of V, being less than V By omitting electrode 8 andusing electrode 7 as the source of desired" secondary electrons thespike" at the end of the V versus ion-current curves can be removed.However the performance of the so-modified apparatus is much inferior tothat hereinbefore described in respect of the reduction factor fornormal ion beams, viz about 2000 compared with 6000 for the argon peakat (m/e) 40 as shown in FIG. 4, so this modified form of the apparatusis not preferred.

Although described with reference to its use with a singlestage massspectrometer, the present invention is also applicable to two-stageinstruments, i.e. instruments in which the magnetic stage is preceded byan electrostatic stage, for example using Nier-Johnson geometry.

We claim:

1. Ion beam intensity measuring apparatus suitable for use with a massspectrometer comprising a first apertured electrode for admitting theion beam and having a surface capable of emitting secondary electrons, afirst retarding electrode located beyond said first apertured electrodeto apply a retarding electric field to ions passing through theaperture, a detector for detecting secondary electrons emitted from saidfirst apertured electrode by ions repelled to said first aperturedelectrode by said field, and a connection for applying a variablepotential to said first retarding electrode.

2. Apparatus as claimed in claim I wherein said secondary electrondetector is mounted on said first retarding electrode facing said firstapertured electrode.

3. Apparatus as claimed in claim 2 wherein the surface of said secondaryelectron detector facing said first apertured electrode includes acoating which is substantially opaque to said ions but substantiallytransparent to said secondary electrons.

4. Apparatus as claimed in claim 3 wherein said coating is a metalliclayer.

5. Apparatus as claimed in claim 3 wherein said detector is atranslucent scintillator and forms the central portion of said firstretarding electrode.

6. Apparatus as claimed in claim 5 wherein the photocathode of aphotomultiplier tube is located beyond said first retarding electrodeopposite that surface of said scintillator which faces away from saidfirst apertured electrode.

7. Apparatus as claimed in claim 2 wherein said first aperturedelectrode is preceded by a second retarding electrode having an apertureand having a connection for applying thereto a variable potential lessthan the potential applied to said first retarding electrode.

8. Apparatus as claimed in claim 7 wherein said second retardingelectrode includes a grid.

9. Apparatus as claimed in claim 2 wherein said first aperturedelectrode is preceded by a second apertured electrode arranged to beheld at the same potential as said first apertured electrode and whereinan apertured electron-suppression electrode is located between saidfirst and second apertured electrodes.

10. Apparatus as claimed in claim 7 wherein said first aperturedelectrode is preceded by a second apertured electrode arranged to beheld at the same potential as said first apertured electrode, wherein anapertured electron-suppression electrode is located between said firstand second apertured electrodes, wherein said second retarding electrodeprecedes said second apertured electrode, and wherein a third aperturedelectrode precedes said second retarding electrode and is arranged to beheld at the same potential as said first and second aperturedelectrodes.

11. Apparatus as claimed in claim 2 wherein said first retardingelectrode and said first apertured electrode are substantially parallelto one another.

12. A method of measuring a metastable fragment ion spectrum with a massspectrometer comprising admitting the ion beam through an aperturedelectrode having a surface capable of emitting secondary electrons,applying a potential to a retarding electrode located beyond theapertured electrode to repel ions back to said apertured electrode andthereby produce secondary electrons thereat, allowing said secondaryelectrons to impinge on a detector mounted on said retarding electrode,and varying the potential applied to said retarding electrode between apotential at which substantially all ions in the beam are repelled tosaid apertured electrode and a potential at which substantially onlymetastable fragment ions in the beam are so repelled.

13. A method of measuring the intensity of ions in a beam within adesired energy range comprising causing the ion beam to approach anapertured retarding electrode, applying a potential to said aperturedretarding electrode to repel ions below a selected energy approachingsaid apertured retarding electrode, admitting those ions which traversesaid apertured retarding electrode through a further apertured electrodehaving a surface capable of emitting secondary electrons, applying apotential to a further retarding electrode located beyond said aperturedelectrode to repel ions below a further selected energy back to saidapertured electrode and thereby produce secondary electrons thereat,allowing said secondary electrons to impinge on a detector mounted onsaid further retarding electrode, the potential applied to said furtherretarding electrode being greater than that applied to the aperturedretarding electrode and said potentials being selected to define betweenthem the desired ion energy range.

1. Ion beam intensity measuring apparatus suitable for use with a massspectrometer comprising a first apertured electrode for admitting theion beam and having a surface capable of emitting secondary electrons, afirst retarding electrode located beyond said first apertured electrodeto apply a retarding electric field to ions passing through theaperture, a detectOr for detecting secondary electrons emitted from saidfirst apertured electrode by ions repelled to said first aperturedelectrode by said field, and a connection for applying a variablepotential to said first retarding electrode.
 2. Apparatus as claimed inclaim 1 wherein said secondary electron detector is mounted on saidfirst retarding electrode facing said first apertured electrode. 3.Apparatus as claimed in claim 2 wherein the surface of said secondaryelectron detector facing said first apertured electrode includes acoating which is substantially opaque to said ions but substantiallytransparent to said secondary electrons.
 4. Apparatus as claimed inclaim 3 wherein said coating is a metallic layer.
 5. Apparatus asclaimed in claim 3 wherein said detector is a translucent scintillatorand forms the central portion of said first retarding electrode. 6.Apparatus as claimed in claim 5 wherein the photocathode of aphotomultiplier tube is located beyond said first retarding electrodeopposite that surface of said scintillator which faces away from saidfirst apertured electrode.
 7. Apparatus as claimed in claim 2 whereinsaid first apertured electrode is preceded by a second retardingelectrode having an aperture and having a connection for applyingthereto a variable potential less than the potential applied to saidfirst retarding electrode.
 8. Apparatus as claimed in claim 7 whereinsaid second retarding electrode includes a grid.
 9. Apparatus as claimedin claim 2 wherein said first apertured electrode is preceded by asecond apertured electrode arranged to be held at the same potential assaid first apertured electrode and wherein an aperturedelectron-suppression electrode is located between said first and secondapertured electrodes.
 10. Apparatus as claimed in claim 7 wherein saidfirst apertured electrode is preceded by a second apertured electrodearranged to be held at the same potential as said first aperturedelectrode, wherein an apertured electron-suppression electrode islocated between said first and second apertured electrodes, wherein saidsecond retarding electrode precedes said second apertured electrode, andwherein a third apertured electrode precedes said second retardingelectrode and is arranged to be held at the same potential as said firstand second apertured electrodes.
 11. Apparatus as claimed in claim 2wherein said first retarding electrode and said first aperturedelectrode are substantially parallel to one another.
 12. A method ofmeasuring a metastable fragment ion spectrum with a mass spectrometercomprising admitting the ion beam through an apertured electrode havinga surface capable of emitting secondary electrons, applying a potentialto a retarding electrode located beyond the apertured electrode to repelions back to said apertured electrode and thereby produce secondaryelectrons thereat, allowing said secondary electrons to impinge on adetector mounted on said retarding electrode, and varying the potentialapplied to said retarding electrode between a potential at whichsubstantially all ions in the beam are repelled to said aperturedelectrode and a potential at which substantially only metastablefragment ions in the beam are so repelled.
 13. A method of measuring theintensity of ions in a beam within a desired energy range comprisingcausing the ion beam to approach an apertured retarding electrode,applying a potential to said apertured retarding electrode to repel ionsbelow a selected energy approaching said apertured retarding electrode,admitting those ions which traverse said apertured retarding electrodethrough a further apertured electrode having a surface capable ofemitting secondary electrons, applying a potential to a furtherretarding electrode located beyond said apertured electrode to repelions below a further selected energy back to said apertured electrodeand thereby produce secondary electrons thereat, allowing said secondaryelectrons to impinge oN a detector mounted on said further retardingelectrode, the potential applied to said further retarding electrodebeing greater than that applied to the apertured retarding electrode andsaid potentials being selected to define between them the desired ionenergy range.